Mainspring Barrel

Specification

Technical parameters

Overview

The barrel is a drum with teeth on its outer edge containing a coiled mainspring. The spring's inner end is hooked to the arbor, the outer end to the barrel wall. Winding turns the arbor and coils the spring tighter; running lets the barrel rotate and unwind it. The barrel teeth drive the centre wheel, and from there the rest of the train.

Modern automatic springs end in a slipping bridle — a short, thicker section that grips the barrel wall by friction until the spring is fully wound, then slips. It is the part that makes overwinding impossible, and it is also the part that fails most often: a bridle that slips too early gives a short reserve, one that has welded itself to the wall can be felt as a hard stop when winding.

Older hand-wound movements use a hooked spring with no bridle. Those genuinely can be overwound, and a spring that breaks under a hard wind can damage the barrel teeth and the centre wheel pinion in one event.

What matters about it

• Stores all the energy the movement uses
• Slipping bridle prevents overwinding damage
• Torque falls across the reserve, which is why rate drifts
• Greased rather than oiled — the loads are too high for oil
• Barrel teeth drive the going train directly

Practical notes

• Grease the barrel wall, not the spring — an over-greased spring drags and wastes torque.
• A spring that coils unevenly or cones is tired; replace it rather than reusing it.
• Check barrel endshake before reassembly; vertical play here changes amplitude in every position.
• Modern pre-lubricated sealed barrels are not serviceable — replace the unit rather than opening it.

Compatible with